The specific aims of this proposal are 1. To develop an HIV-based gene transfer system and 2. To compare the gene transfer efficiencies between murine-based systems and the HIV-based system. In addition, this system will be used to identify determinants of toxicity associated with HIV expression. Murine-based retroviral vectors are currently being used for gene transfer in basic research as well as in gene therapy protocols. However, these systems are limited by their inability to transduce nondividing cells, such as terminally differentiated macrophages and human hematopoietic stem cells (HSC), and by their inefficient transduction of human primary cells. The development of a human immunodeficiency virus (HIV)-based gene transfer system could potentially solve these problems. HIV-1, as a lentivirus, has the ability to infect nondividing cells and any human primary cell that expresses the cell surface receptor CD4. Currently, there are no HIV-based gene transfer systems available for gene therapy. Problems encountered during the development of these systems have included low virus titers, limited tropism, and the inability to make stable producer lines. THis proposal presents the ongoing development of an inducible HIV based packaging system with enhanced tropism and increased titers through the use of various envelope pseudotypes.